In wireless communication systems, the quality of the signal depends in large part on the amount of noise measured at the receiver antenna. In these systems, the noise figure is the ratio of the output noise power to the thermal noise in the input termination at standard noise temperature. The noise figure thus represents the ratio of actual output noise to that which would remain if the device itself was noise free, and provides an indication of the performance of a radio receiver. The noise power is typically used to denote the cumulative effects of noise figure at the receiver and the ambient (e.g. non-system) interference. The knowledge of noise power at the receiver is crucial for several blocks in the transceiver chain, which include, but are not limited to demodulation, decoding, power control, link adaptation, and similar operations.
In general, there are three main sources of noise at the receiver antenna: (1) ambient (non-system) noise or interference which is at or near the same operating frequency range of the desired signals; (2) circuit noise, which is noise introduced or picked up by the circuits or blocks in the RF (radio frequency) stage of the receiver itself; and (3) system or system-like interference, which is introduced by other transmitters or sources of desired signals for other receivers, but not for a particular receiver. In order to design and build effective wireless receivers, it is important to know or at least be able to accurately estimate the noise power at the receiver. However, all of the interference at the receiver may factor into the noise power calculation, including interference from other transmitters, which is technically not noise, but rather system or system-like interference. It is important, therefore, to separate the ambient noise and the circuit noise from the system or system-like interference, in order to obtain a true estimation of noise power at the receiver.
In wireless communication systems, noise power is commonly estimated by measuring the received signal power in time and/or frequency slots that are explicitly not used for data transmission in the system. Those slots are typically known as guard times and guard bands, in time and frequency domain, respectively. In many situations, guard times and guard bands are either not available or are otherwise congested with adjacent channel interference, which would contribute to unrealistically high noise power levels if measured using conventional methods. One example of a current system which is susceptible to such inaccurate noise power measurements is the IEEE 802.16e standard and its Wimax profile version (IEEE P802.16-2004/Cor1/D5).
It has been widely asserted that noise power at the receiver cannot be measured on the pilot signals in the IEEE 802.16e standard, however it is desirable to utilize such guard bands to separate ambient noise from system-like signals to measure the actual noise power at the input stage of a receiver.